1. Field of the Invention
This invention relates to a process of graphitizing carbon bodies which are forced together in end-to-end alignment to form a horizontal train, which is axially conveyed through a graphitizing furnace in feeding steps, which are at least as long as one carbon body, and in said furnace is heated to a graphitizing temperature at least in part of its length by a supply of electric current, between consecutive conveying steps said train is shortened at the exit end of the furnace by the removal of at least one graphitized carbon body and is lengthened at the entrance end of the furnace by the addition of at least one carbon body which is to be graphitized. The invention relates also to an apparatus for carrying out the process.
2. Description of the Prior Art
Preburnt carbon bodies which are to be graphited are forced together in end-to-end alignment to form a train, which is fed into a graphitizing furnace, in which electric current supplied via electrodes applied to the train is caused to flow through the train so that the carbon bodies can be heated to the required graphitizing temperature by the Joule effect. In order to avoid the presence of an oxidizing atmosphere in the graphitizing furnace, the latter is filled either with an inert gas or with carbon particles. A filling with carbon particles will afford the advantage that the train if it is horizontally oriented will be supported by the carbon particles and cannot sag. But a disadvantage resides in that the carbon particles of the packing will be heated with the train so that more energy will be required. As the temperature of the carbon particle increases, their electrical conductivity will also increase so that conducting paths which are parallel to the train may be established in the packing.
The disadvantages involved in the use of a bed of carbon particles will be avoided if said bed is replaced by an inert gas atmosphere in the graphitizing furnace but in that case the carbon bodies of the train will have to be properly supported by additional means because the train is no longer supported by a particulate packing. Because the train is not supported by a bed of particles in a furnace filled with an inert gas atmosphere and it is difficult to supply electric current to a moving train, a continual movement of the train through the graphitizing furnace has only been disclosed in conjunction with graphitizing furnaces containing a packing of carbon particles (EP-B-0 121,530). In that known process the train is moved through the furnace in steps and after each step is shortened at the exit end of the furnace by the removal of a graphitized carbon body and is lengthened at the entrance end of the furnace by the addition of a carbon body which is to be graphitized. As the train is moved, it is gripped between two gripping heads, which are disposed at opposite ends of the furnace, and the train is axially displaced by said gripping haeds to the extent of the length of one carbon body. Thereafter the gripping heads release the train at its end faces so that the train can be shortened and lengthened, respectively. The train may readily be relieved from the axial compressive stress between consecutive feeding steps because the train is supported by the packing of carbon particles and because the electric current is shut off between the consecutive feeding steps so that the carbon bodies need not be forced together for establishing a conductive connection between adjacent carbon bodies. The carbon particles of the packing will ensure a comparatively simple conduction of current between electrodes, which in the furnace extend radially to the train, and the train. Owing to their elevated temperature the carbon particles have an adequate electrical conductivity adjacent to the electrodes so that current may be supplied via terminals which are so distributed over the length of the train that a distinction can be made between a preheating zone, a graphitizing zone and a cooling zone. Heat sinks protruding into the interior of the furnace in the cooling zone may be provided to effect an additional dissipation of heat from the cooling zone. But the disadvantages involved in the use of a packing of carbon particles in the furnace will not be eliminated in such an arrangement. Besides, the desirable support of the train will increase the resistance opposing the feeding movement of the train and the surface of the train may be damaged by the movement of the train in contact with the carbon particles.